1. Field of the Invention
The present invention relates to a randomly accessible magnetic disk device which can be an external memory device for a computer system and, particularly, to a method for making a number of sectors, the recording and reproducing units, on the disk variable in a radial direction of a disk and a device for performing the same method.
2. Related Art
It is known to provide a magnetic disk device, which is adapted to record and reproduce fixed length data. It is also known to provide a magnetic disk device capable of recording and reproducing variable length data.
In one such conventional magnetic disk device, the amount of data which can be recorded at a specific circumference of the disk defined by a specific radius, is constant across the radius of the disk. As a consequence a data recording density linearly decreases along the radius of the disk as the magnetic recording/reproducing head travels from the inner to the outer radii. In a magnetic disk device having such a recording density distribution in a fixed length data system, the number of sectors, the recording and reproducing units, is uniform along the radial direction of the disk.
Another such conventional device includes a magnetic disk device having a substantially constant recording density across the radius of the disk, or is only slightly reduced along the radius of the disk, as the magnetic recording/reproducing head travels from the inner to the outer radii. As a consequence, the memory capacity of such a device is increased as compared with the first conventional example. U.S. Pat. Nos. 4,799,112 and 4,894,734 each disclose examples of such conventional constant density devices.
The first conventional device has a draw back in that its memory capacity per device is smaller as compared with the second conventional device. However, while the second conventional device has merit in that memory capacity per device can be increased compared with the first conventional device, recording density in radially outward portion of the disk is still slightly lowered compared with that in radially inward portion of the disk as in U.S. Pat. No. 4,799,112 because a recording and reproducing margin in the radially outward portion of the disk is smaller compared with the first conventional example. This presents a different drawback namely that the recording and reproducing margin in radially middle portion of the disk is reduced. The conventional solution to this latter problem is to reduce the memory capacity of the device thereby at least partially nullifying the basic advantage of the second conventional device.